I won't say they "don't exist", but CD & MP3 doesn't support 5.1. AAC supports multiple channels, but as far as I know iTunes downloads are all stereo (or mono). That means it's "difficult" to distribute, sell, buy, or play, and
there is not much supply or demand.
Of course, Blu-Ray and DVD support surround sound and just about everybody can play them, but those formats are rarely used for audio distribution.
You can search for some DVD-Audio or SACD discs (these formats support multi-channel). But, neither of those formats ever "caught-on", the demand never developed, and they are virtually dead formats. And, many DVD-Audio & SACD discs are "high resolution" 2-channel stereo.
...I found
Pink Floyd Dark Side Of The Moon in 5.1 on SACD. (I found it on eBay, so it might be out of print.)
...I've got a shelf-full of DVD concerts and I'd guess that more than half of them have 5.1 channel Dolby or DTS. And, many of them sound freekin' amazing! Of course, there are many popular songs by the original artists on the DVDs, but they are live versions, not the "popular" studio versions.